1. Field of the Invention
The present invention relates to a transmitting apparatus and radio communication apparatus, and more particularly to a transmitting apparatus and radio communication apparatus that perform power amplification of a transmit signal to output that signal.
2. Description of the Related Art
Heretofore, a class A or class AB linear amplifier has been used to linearly amplify an envelope fluctuation component in a high-frequency power amplifier that amplifies a modulation signal containing an envelope fluctuation component. This kind of linear amplifier has excellent linearity, but because it constantly consumes power associated with a DC bias component, so this kind of the linear amplifier has lower power efficiency than a class C, class D, class E or similar nonlinear amplifier. Consequently, when this kind of high-frequency power amplifier is applied to a portable radio device that uses a battery as its power source, usage time has been shortened due to the large power consumption of the high-frequency power amplifier. Also, when this kind of high-frequency power amplifier is applied to a base station apparatus of a radio system in which a plurality of high-power transmitting apparatuses are installed, this has resulted in a large apparatus size and increased heat generation.
Thus, the transmitting apparatus 1 employing a polar modulation method shown in FIG. 1 has been proposed as a high-efficiency transmitting apparatus. This transmitting apparatus 1 is equipped with an amplitude phase conversion section 2, an amplitude modulation signal amplifier 3, a frequency synthesizer 4, and a high-frequency power amplifier 5, which is a nonlinear amplifier.
A baseband modulation signal 20 is input to amplitude phase conversion section 2. A baseband amplification modulation signal 21 output from amplitude phase conversion section 2 is input to amplitude modulation signal amplifier 3. A baseband phase modulation signal 23 output from amplitude phase conversion section 2 is input to frequency synthesizer 4. A high-frequency phase modulation signal 24 output from frequency synthesizer 4 is input to high-frequency power amplifier 5. A transmit output signal 25 is output by high-frequency power amplifier 5.
Next, the operation of transmitting apparatus 1 will be described. First, if baseband modulation signal 20 is designated Si(t), this baseband modulation signal Si(t) can be expressed by Equation 1 below.Si(t)=a(t)exp[jφ(t)]  (1)
Here, a(t) is amplitude data and exp[jφ(t)] is phase data. Amplitude data a(t) and phase data exp[jφ(t)] are extracted from baseband modulation signal Si(t) by amplitude phase conversion section 2. Amplitude data a(t) corresponds to baseband amplification modulation signal 21, and phase data exp[jφ(t)] corresponds to baseband phase modulation signal 23. Amplitude data a(t) is amplified by amplitude modulation signal amplifier 3 and provided to high-frequency power amplifier 5. By this means, the power supply voltage of high-frequency power amplifier 5 is set based on amplitude data a(t).
Frequency synthesizer 4 modulates carrier angular frequency ωc with phase data exp[jφ(t)] and generates high-frequency phase modulation signal 24, which is input to high-frequency power amplifier 5. Here, if high-frequency phase modulation signal 24 is designated Sc, high-frequency phase modulation signal Sc can be expressed by Equation 2 below.Sc=exp[ωct+φ(t)]  (2)
By using a nonlinear amplifier for high-frequency power amplifier 5 in this way, a signal in which a power supply voltage value based on amplitude data a(t) of high-frequency power amplifier 5 and the output signal of frequency synthesizer 4 are multiplied together is generated amplified by gain G of high-frequency power amplifier 5. High-frequency power amplifier 5 outputs this amplified generated signal as transmit output signal 25. Here, if transmit output signal 25 is designed RF signal Srf, this RF signal Srf can be expressed by Equation 3 below.Srf=Ga(t)Sc=Ga(t)exp[ωct+φ(t)]  (3)
The signal input to high-frequency power amplifier 5 is a phase modulation signal that does not have an amplitude direction fluctuation component, and is thus a fixed envelope signal. Therefore, an efficient nonlinear amplifier can be used as high-frequency power amplifier 5, enabling a highly efficient transmitting apparatus 1 to be implemented. A transmitting apparatus employing a polar modulation transmitting method of this kind is described, for example, in Japanese Patent Publication No. 2002-530917 and Japanese Patent Publication No. 2004-501527.
However, with above-described transmitting apparatus 1, since high-frequency power amplifier 5 is a nonlinear amplifier, when the output power of high-frequency power amplifier 5 is controlled the output signal does not vary linearly with respect to the input signal. Therefore, the power supply voltage has to be varied in output signal control in the same way as in amplitude modulation. In this case, the output power control range has been limited by leakage power, transistor operating limits with respect to the power supply voltage, and so forth. There has also been a problem of error with respect to the required transmission power due to characteristic changes resulting from variations in electronic components of the transmitting apparatus, temperature fluctuations, and so forth.